Iran's Revolutionary Guards have bought a controlling stake in the country's telecommunications company, fuelling suspicions that the organisation is quietly staging a military takeover.

Tose'e Etemad Mobin, a consortium controlled by the guards, paid £5bn for a 51% stake under a privatisation scheme. It was claimed that a rival enterprise had been unfairly excluded from the bidding process because it lacked appropriate "security qualifications".

Critics have warned that the deal exposes ordinary people, especially political activists, to intensified spying and electronic surveillance.

The news came days after the governor of Iran's central bank, Mahmoud Bahmani, announced that a finance company owned by the Revolutionary Guards, the Ansar Institute, had been cleared to become a fully fledged bank.

The Revolutionary Guards – formed in 1979 to safeguard the Islamic revolution – have built a financial empire with interests including oil and gas fields, airports and eye and dental clinics during the presidency of Mahmoud Ahmadinejad, himself a former member. The empire has been awarded building and engineering contracts worth billions of pounds and is thought to control the smuggling of contraband into Iran.

Its own high street bank would be the most public demonstration yet of the inroads the institution has made into the country's economy. Ansar was originally established to give credit-free loans to the poor but was upgraded to become a recognised financial credit organisation after Ahmadinejad's first election victory in 2005. The president rejected complaints from economists that it was not subject to proper controls.

"This new bank is one of many recent steps intended to complete the dominance of the Revolutionary Guards in Iran," said Jamshid Assadi, an Iranian economist based at the ESC Groupe business school in Dijon, France. "They are now very active, indeed dominant, in many sectors of the Iranian economy. They were not involved in oil and gas before but now under Ahmadinejad, they are."

The telecoms takeover has provoked accusations that the government's privatisation programme – required under Iran's constitution – is a sham designed to sell state assets to the Revolutionary Guards.

The Expediency Council, an influential state body headed by Hashemi Rafsanjani, Ahmadinejad's most powerful political rival, has promised to investigate the purchase, along with other privatisations conducted over the past two years.

Mohammad Nourizad, a pro-fundamentalist journalist who broke with the hardliners over the bloody crackdown that followed Ahmadinejad's disputed re-election last June, warned that it would be used to step up monitoring of the government's opponents.

"Getting access to telecommunications management has always been vital for the security requirements of the revolutionary guards and the iron men behind the scenes," he wrote in a blog. "It means control over the country's entire telecommunications system, including landline telephones, mobiles, text messages, the internet and any other stuff linked to telecommunications. After that, it's a piece of cake … to trace people."

The authorities are widely believed to have used telecommunications technology to keep tabs on post-election protest organisers by intercepting mobile calls and text messages and by hacking into email accounts.